1. Field of the Invention
This invention relates to aircraft automatic flight control systems, and more specifically to those systems which provide automatic stabilizer trim control.
2. Background of the Invention
Automatic flight control systems which provide automatic trim control are well known in the art, an example of which is disclosed in U.S. Pat. No. 2,845,239 assigned to the Applicants' assignee. In older aircraft, pitch trim control was ordinarily provided by a trim tab on an elevator which was deflected relative to a fixed horizontal stabilizer, whereas in most modern transport type aircraft, long term trim control is provided by a rotatable stabilizer having an elevator deflected relative thereto for providing short term pitch attitude control. Once a statically stable aircraft has been trimmed to a constant angle of attack normally by an upward, i.e., trailing edge up adjustment of the stabilizer to maintain lift equal to weight, it will tend to return to the trimmed angle of attack whenever it is disturbed in pitch through manual or automatic operation of the elevator. During sustained flight in the automatic pilot cruise mode, i.e., constant altitude cruise, a substantial amount of fuel is burned off, and the resultant weight loss will require a corresponding reduction in lift and possible movement of the aircraft center of gravity relative to the center of lift. Accordingly, the angle of attack will normally be gradually decreased to decrease lift and balance any lift pitching moments. This is normally accomplished through some form of automatic trim control loop in the autopilot system. In a typical automatic trim system, when the altitude control signal calls for down elevator and a resultant down elevator position feedback signal, and when the latter signal exceeds a predetermined high (auto trim pull-in) threshold value for a predetermined period of time (to distinguish from short term pitch attitude stabilization), a trim motor is actuated which adjusts the stabilizer's trailing edge in a downward direction (from its previous position). This results in an altitude deviation signal of a polarity to drive the elevator toward its zero position. When the elevator position feedback signal reduces to a predetermined low (auto trim drop-out) value the trim motor is stopped. Normally, the trim initiation threshold is varied as a function of airspeed to provide desired control surface effectiveness. Of course, automatic trim is necessary in both directions to compensate for aircraft load changes in both directions but the present invention is primarily concerned with changing trim conditions associated with decreasing angle of attack during long term cruise fuel burn-off. The elevator trim deflection threshold pull-in and drop-out limits cannot be too low since aircraft stability would be adversely affected especially during low speed, low level flight conditions, nor can the threshold limits be too high since objectional pitching transients would be induced when the automatic flight control system in disengaged. In the present invention, the normal elevator trailing edge up trim thresholds are not changed because trailing edge up only adds pitching moment to stabilizer-induced pitching moment and does not represent unnecessary drag.
When the elevator is deflected relative to the stabilizer, there is always some aerodynamic drag. However, whenever the elevator is deflected in a direction to oppose pitching moments produced by the stabilizer, unnecessary aerodynamic drag is produced. Thus, a throttle increase and hence increased fuel flow is necessary to maintain cruise airspeed. Heretofore, this increased throttling and attendant increase in fuel consumption have been considered acceptable, but now significant savings in fuel costs may be realized by reducing this unnecessary increase in aerodynamic drag. A flight crew may reduce this undesirable drag manually by visually monitoring the elevator deflection on a meter and periodically setting any trim error to zero or substantially at zero before the normal trim threshold limit is exceeded. Such manual periodic reduction of trim error is both inefficient and a nuisance to the flight crew who may be distracted from their regular cockpit duties. Accordingly, there is a need to provide the aviation industry with a simple and efficient solution to the problem of reducing the increased and unnecessary drag induced by conventional methods of automatic stabilizer trim control such that significant savings in fuel costs may be realized.